Recently, there has developed great interest in compressors which lack traditional moving parts. Such compressors are particularly suitable for application in refrigeration systems which use non-ozone depleting refrigerants as the working medium. One type of such compressor is the so-called acoustic compressor whereby a standing acoustic wave is generated in a chamber, containing the working medium. The pressure variations caused by the acoustic wave are then utilized to pump, and therefore compress, the medium. Acoustic compressors are described in detail in U.S. Pat. No. 5,020,977 issued to Lucas, the disclosure of which is hereby incorporated by reference.
In the acoustic compressors described in the aforementioned '977 patent, the acoustic waveform is generated by a flexible diaphragm mounted at one end of the chamber containing the working medium and driven by an electromagnetic actuator. The arrangement is thus similar to a conventional loudspeaker. The frequency at which the diaphragm is driven is chosen in accordance with the physical properties of the working medium and the geometry of the chamber so that a standing wave (i.e., one having stationary nodes and antinodes) is produced within the chamber. That is, the diaphragm is driven at a frequency corresponding to one of the normal modes of the system so that a condition of resonance is achieved within the chamber.
Obviously, it is advantageous in a refrigeration system for the acoustic compressor to generate pressures as high as possible in order to compress the refrigerant. At a given driving frequency, the only way to increase the pressure amplitude of the acoustic waveform is to increase the amplitude of the diaphragm's back and forth excursion. Such an increase is severely constrained, however, by the physical characteristics o both the flexible diaphragm and the electromagnetic actuator which permit only a limited range of motion at a given frequency. Furthermore, a flexible diaphragm can only withstand a limited amount of pressure before it physically fails.